1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile and the like, utilizing an electrophotographic process.
2. Related Background Art
Among image forming apparatuses utilizing an electrophotographic process, in a type in which a transfer belt is used as a transfer means for transferring a toner image onto a transfer material at a transfer station, the transfer belt has a function for transferring the toner image onto the transfer material, a function for separating the transfer material after the transferring and a function for conveying the separated transfer material.
Generally, since a peripheral or circumferential length of the transfer belt is smaller than a length of the transfer material in a conveying direction thereof, when the toner image is transferred onto the transfer material, it is insufficient to rotate the transfer belt by one revolution, and second revolution of the belt also contributes to the transferring. Thus, in order to prevent "charge-up" phenomenon, in which charges having polarity opposite to polarity of transfer bias applied to the transfer belt during the first revolution of the belt are accumulated on the surface of the transfer belt, from affecting a bad influence upon the image, constant-current-controlled transfer bias has been applied to the transfer belt.
In the conventional image forming apparatuses in which the constant current is used as the transfer bias for the transfer belt, if resistance of the transfer material itself is increased due to low humidity environment, when the toner (developer) image is transferred onto the transfer material, for example, in an image forming apparatus having an intermediate transfer member 7 and a transfer belt 8 as shown in FIG. 8, if an area of a secondary transfer station (transfer nip) of the intermediate transfer member 7 (which is contacted with the transfer belt 8 and at which the firstly-transferred toner image T is secondary-transferred onto the transfer material) is great, the secondary transfer station (transfer nip) extends over both the transfer material P and the intermediate transfer member 7, with the result that, at a rear end part P1 (downstream end in a conveying direction) of the transfer material P (which part is a non-image area where the toner image is not existed), load impedance is suddenly decreased to reduce transfer voltage (having positive polarity), thereby reducing an amount of charges (having positive polarity) applied to the back surface of the transfer material P.
Incidentally, the firstly transfer bias having positive polarity is applied to the intermediate transfer member 7 and the toner T to be transferred to the transfer material P is charged with negative polarity. Further, in FIG. 8, at the secondary transfer station (transfer nip), although the intermediate transfer member 7 is shown to have a flat configuration, the intermediate transfer member is actually formed as a drum.
As mentioned above, at the rear end part P1 (non-image area) of the transfer material P, since the amount of the charges (having positive polarity) applied to the back surface of the transfer material P is reduced, as shown in FIG. 8, the charges (having positive polarity) for holding the toner T (having positive polarity) applied to the back surface of the transfer material P is partially reduced.
As a result, after the transferring, when the transfer material P is separated from the transfer belt 8, peel discharge is generated between the transfer material P and the transfer belt 8. Consequently, at the rear end part P1 of the transfer material P having the reduced charges for holding the toner, as shown in FIG. 9, the toner is scattered due to discharge shock.